Pickup tube target structure



S. A. ocHs PICKUP TUBE TARGET STRUCTURE Filed Dec. 21, 1959 Dec. 4, 1962 Airmail-7 3,957,348 Patented Dec. 4, 1962 3,067,348 PICKUP TUBE TARGET STRUC Stefan Albert Ochs, Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Dec. 21, 1959, Ser. No. 860,988 7 Claims. (Cl. 313-67) This invention relates to target structures for use in pickup or camera tubes. In particular, this invention relates to two-sided target structures for use in photoemissive type pickup tubes.

During operation of a conventional image orthicon type camera tube, photoemission, in proportion to the light from a scene to be reproduced, is directed onto one side of a semi-conducting type glass target. to reaching the target, the photoelectrons pass through an apertured mesh screen or collector electrode. When the photoelectrons land on the target, these photoelectrons produce a potential variation or stored image on the opposite side of the target corresponding to the photoelectron image. An electron beam is then used to scan the stored image, which removes the stored image, and produces an output signal in proportion to the original photoelectron image.

The targets used for image orthicon type camera tubes prior to this invention have been made of a thin membrane of lime glass. It has been found that, when using targets of this type, the resistivity of the target increases somewhat as the tube is operated. The cause of this increase in resistivity is not known completely but may, in part, be due to a depletion of the positive ions on the image side of the target. As the resistivity increase occurs, excessive sticking or picture retention occurs which limits the useful life of the tube.

In the prior art tubes of this type, it has been found that the tube is highly sensitive to mechanical movement such as shock or vibration. One of the reasons for this sensitivity is that the target and the mesh screen are two stretched membranes that are mounted very close together and parallel to each other. Vibration of the tube may therefore cause variations in the spacing between the two membranes, thus causing variations in the electrical capacitance between the two membranes. The capacitance variations cause fluctuations or microphonics in the output electrical signal.

It has been suggested that a target structure be constructed using a conducting plate having a plurality of apertures therethrough, with insulating material positioned in each aperture. One of the difliculties of this type of target structure is that the number of insulators, or conductors surrounded by insulators, that can be provided in a given area is low as compared to the number that are desired for high picture definition. A still further difiiculty is that both conducting surfaces and insulating surfaces are exposed to the photoelectron image. Because of this exposure, the insulating surfaces tend to build up a potential difference between the two surfaces of the target and, after a change in the configuration of the charge image, this potential difference requires an appreciable length of time before it has decayed. This effect tends to result in objectionable varitions in the output signal or stored image.

Other mosaic type targets suggested in the prior art include the use of highly insulating members, such as mica, through which a plurality of holes have been drilled and which have been filled with conducting plugs. These targets have also suffered from lack of picture definition due to the small number of plugs per unit of area that can be drilled. Furthermore, in the known structures of this general type, the insulating material is exposed to beam bombardment resulting in spurious charges.

Just prior In still further types of targets that are known in the prior art a sufficiently large number of conductors per unit area have been provided. However, these structures have been used with the conventional mesh collector electrode screens and the problem of microphonics has remained.

It is therefore an object of this invention to provide an improved target electrode assembly for use in an image orthicon type pickup tube.

It is a further object of this invention to provide an improved pickup tube characterized in its substantial freedom from microphonics and its high picture definition.

These and other objects are accomplished in accordance with this invention by providing a novel pickup tube including a sub-assembly sheet comprising a semiconducting oxidized metal, having a large number of conducting plugs extending partway therethrough, and with a mesh collector electrode attached to the oxidized metal in such a manner as to be spaced from the conducting plugs. The integral target and mesh electrodes are arranged so that, although the mesh electrode is electrically insulated from the conducting plugs by means of the oxidized metal, only electrode surfaces are exposed to the photoelectron beam.

The invention will be more clearly understood by reference to the accompanying single sheet of drawings wherein:

FIG. 1 is a transverse sectional view of an improved image orthicon tube utilizing this invention;

FIG. 2 is an enlarged fragmentary plan view of the target shown in FIG. 1; and

FIG. 3 is a sectional view of the target shown in FIG. 2.

Referring now to the figures in detail, and specifically FIG. 1, an image orthicon tube 10 is shown. The image orthicon tube 10 comprises an evacuated envelope 12 having an electron gun 14 in one end thereof. Surrounding the electrode gun 14 is an electron multiplier structure 16. The electron gun 14 is designed to produce an electron beam 18 which is scanned over one side of a novel target electrode 20.

- On the opposite side of the target electrode 20, and within an enlarged end of the envelope 12, there is an image section 22. The image section 22 comprises a photoemissive cathode 24 and its associated electrodes. During operation of the tube 10, the photocathode 24 produces an electron image corresponding to the light from a scene to be reproduced. The photoelectron image is directed onto the target assembly 20.

The particular elements that have been described, other than the novel target assembly 20, may be conventional and form no part of this invention except for their cooperation with the novel target 20. If desired, a more detailed description of an image orthicon pickup tube may be obtained by referring to one or more of many issued patents, such as Weimer Patent No. 2,537,250, Law Patent No. 2,460,093, or Weimer Patent No. 2,433,- 941.

' As was previously pointed out, the image orthicon type pickup tubes using'the prior art targets, such as lime glass, have a tendency to retain a picture after a certain period of tube operation. Thus, the life of these tubes is limited by picture sticking. Two of the factors which are believed to be significant in the appearance of excessive sticking are the operating temperature and the ionic conduction of the glass target.

In accordance with this invention, the target 20 comprises a mosaic-like plug type structure. The target 20, which is shown more clearly in FIGS. 2 and 3, includes a thin metal member 30 which has been oxidized, preferably by anodizing, so that the metal member 30 is an electrical insulator or a highly resistive type semiconductor. The oxidized metal member 30 should have a resistivity of at least ohm-centimeter, in order that the charge deposited on any one conducting plug, to be described, will not leak onto any neighboring plugs in such a way as to impair the resolution of the charge image stored on the target. The member 30 may be made entirely of a material such as aluminum oxide, magnesium oxide or other metal oxide which is strong enough to be self-supporting while resistive enough to prevent spreading of the electrical charges.

In the metal oxide member 30 there is provided a relatively large plurality of extremely fine metal plugs 32. The metallic plugs, or buttons, 32 comprise an electrically conductive material, such as copper or nickel, which may be plated into the apertures in the insulating oxidized metal 30. The metal plugs 32 form the only electrical conductive paths from one side to another of the target 20. For proper picture definition, a relatively large number of plugs per inch, each electrically insulated from the other, is required. Targets having as many as 1,500 plugs per'linear inch have been made by the subsequently described process. Targets having approximately 750 plugs per linear inch or greater have been found to operate satisfactorily.

Spaced in a plane slightly removed from the plane of the plugs 32 is a collector grid 34. The collector 'grid 34 comprises an evaporated layer of aluminum, or other conducting metal, which has been deposited on the electrically insulating metallic member. 30 that surrounds the plugs 32. It should be noted that the collector grid 34 is bonded to the electrically insulating metal oxide member 30 as are the conducting plugs 32. Because of this configuration, relative movement of the collector grid 34 and the conducting plugs 32 is eliminated and microphonics are susbtantially avoided.

It should further be noted that no areas of the electrically insulating metal oxide member 30 are exposed directly to the photoelectron image. In other Words, the photoelectrons see either the collector grid 34 or the conducting plugs 32 and not the insulating oxidized metal; Thus, charges will not be developed on the electrical insulator 30.

The target 20 may be manufactured by anodizing, or otherwise oxidizing, a portion of aluminum sheet to provide a metal oxide to a depth of approximately fifteen microns. Then, using a photoresist material on the sheet and a negative of a shadow mesh (not shown), a pattern of holes is exposed on the resist through the mesh and then the oxide is etched completely through the anodized portion of the aluminum sheet. The resultant holes are then partially filled with metal, such as nickel, by any known means, such as plating, to form the metal plugs 32. The base portion, or un-anodized part, of the alt! minum plate is then removed from the aluminum oxideplug structure. This may be done by dissolving the al-uminum sheet in a dilute hydrochloric acid bath. This dissolving process is continued until only the aluminum oxide portion 30 remains which includes the conducting metal plugs 32. The aluminum oxide sheet 30 is translucent due to its thinness. Thus, when the sheet 30 becomes, translucent during the etching process, the aluminum has been completely dissolved away and the process is stopped. One method of manufacturing the aluminum oxide-plug structure may be that described in the copending application Serial Number 721,904, filed March 17, 1958, by C. H. F. Morris, and assigned to the same assignee as the assignee of this application.

After the aluminum oxide-plug structure has been manufactured, it is mounted onto'a glass ring 36 (FIG. 1) with a suitable glass frit, e.g. lead borosilicateglass. At this stage in the manufacturing process, the target 20 and the ring 36 are baked in air at approximately 450 C. for approximately one hour in order to completely remove the photoresist material. After the bake, aluminum is evaporated onto the side of the target 20 that will be positioned toward the photocathode 24. The aluminum evaporation forms the collector grid 34 and also places another layer of conductor 38 (FIGS. 2 and 3) onto the conducting plugs 32. Thus the conductor 34 is a continuous mesh and, during tube operation, functions as a collector grid. It should be noted that the collector grid 34 is insulatingly spaced from the conducting plugs 32 by means of the oxidized metal member 30.

One advantage of the conducting layer 38 is that the evaporated aluminum 38 increases the secondary electron emission ratio of the conducting plugs 32, thereby improving the overall sensitivity of the tube. Other metals can be used for collector grids 34 such as gold or copper. Electrical connection is made to the collector grid 34 by means of a target ring 40 (FIG. 1) to which electrical potentials can be applied.

During operation of the device 10 shown in FIG. l, photoelectrons from the photocathode 24 are directed onto one side of the plug type target 20 by conventional focusing and accelerating means such as those shown in FIG. 1. The photoelectron image lands on the coating 38 on the individual plugs 32 and, by means. of secondary electron emission, produces a charge pattern on the coated plugs 32. Due to the large number of plugs per unit of area, a high definition charge image is formed. When the electron beam scans the opposite side of the charged plugs 32, electrons are deposited to neutralize the charge image. Once the charge has been neutralized, no further electrons land on the target and the beam returns toward the electron gun to be multiplied and thus produce an output, signal.

Due to the close spacing betweenthe collector grid 34 and the metal plugs 32 the target capacitance for a target diameter of about one to two inches is approximately micromicrofarads. Due to the fact that the grid 34 is fixed to, or integral with, the insulating member 30, as are the metal plugs; 32, microphonics, due to the physical vibration of the device 10 are substantially completely eliminated.

What isclaimed is:

1. 'A pickup tube comprising an evacuated envelope having a photocathode in one end thereof, an electrongun in the other end thereof, a target electrode within said envelope and between said photocathode and said electron gun, said target electrode comprising an oxidized metal member, said member having a plurality of apertures extending therethrough, a plurality of electrically conducting plugs each in a different one of said apertures, the length of said plugs being substantially less than the thickness of said member, one end of said plugs being substantially coplanar with one side of said member, andan electrically conductive grid fixed to'the other side of said member, and said grid having a plurality of apertures therethrough each being in registry with a different one of said conducting plugs.

2. A pickup tube comprising an elongated envelope, an electron gun in one end of said envelope, a photocathode iu the other end of said enevlope, a target electrode between said electron gun and said photocathode, said target electrode comprising a membrane of oxidized metal having a plurality of apertures extend ng therethrough, a plurality of conductors each in a different one; of said apertures, and a conducting mesh screen on said oxidized metal and insulated from said conductors, said conducting mesh screen having apertures in registration with said conductors.

3. A pickup tube as in claim 2 wherein said membrane is made of aluminum oxide.

4. A pickup tube as in claim 2 wherein the number of said apertures per linear inch is at least 750.

5. A pickup tube as in claim 2 wherein the resistivity ofsaid oxidized metal is atv least 10 ohm-centimeters.

6. A pickup tube as in claim 2 wherein the thickness of said target is less than 100 microns.

7. A pickup tube as in claim 2 wherein said mesh screen substantially covers said oxidized metal whereby only electrode surfaces are exposed to said photocathode.

References Cited in the file of this patent UNITED STATES PATENTS Zworykin May 2, 1939 Morris May 21, 1957 

